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Change in discounted future net cash flows (Details 7) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items] |  |  |
Beginning of year | $ 883,400 | $ 978,800 | $ 896,100
Net revenues from production | (398,000) | (280,800) | (301,500)
Net change in sales prices and production costs related to future production | 162,200 | (406,300) | 82,300
Extensions and discoveries, net of future production-related costs | 366,500 | 355,300 | 226,300
Improved recovery, net of future production-related costs | 0 | 0 | 0
Purchases of proved reserves, net of future production-related costs | 0 | 0 | 9,500
Sales of proved reserves | (37,800) | (2,600) | 0
Changes in estimated future development costs | 6,700 | 37,600 | 51,100
Development costs incurred during the current year | 141,500 | 77,700 | 56,300
Accretion of discount | 94,600 | 121,400 | 105,000
Net change in income taxes | (141,400) | 110,000 | (55,800)
Revisions of previous estimates | (55,800) | (100,700) | (92,900)
Other | (7,900) | (7,000) | 2,400
Net change | 130,600 | (95,400) | 82,700
End of year | $ 1,014,000 | $ 883,400 | $ 978,800

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